Installing A Powertrax Differential Unit - Making Trax

1/15Double the traction, double the fun. A Powertrax unit had us burning twice the rubber.

Got horsepower? If you are going to do more than go up in smoke, you're gonna need some traction to back it up. The conventional open differentials used in most passenger cars utilize side gears that are splined to the axleshafts and in constant mesh with the differential pinion (spider) gears. These spider gears turn freely on the pinion shafts, and act as idler gears when the rear wheels are turning at different speeds, such as around a corner. This cheap, simple, and effective way to provide differential action in a drive axle has been around for a century. For high performance applications there's a drawback we're all too familiar with-lay on the power, and the conventional differential sends the power to the wheel with the least amount of traction, producing a smoky one-legged burn-out.

Chrysler, like the other manufacturers in the musclecar years, was hip to this requirement, offering their Sure Grip limited-slip differential as an option. With a Sure Grip, the differential was equipped with a clutch mechanism to create locking action to the rear axles which increases as drive torque is applied to the differential. Unfortunately, the vast majority of passenger cars received conventional open differentials. Since the differential case serves as the carrier for the ring gear, installing an OE-style limited slip requires full disassembly of the rear, and the accompanying "setting-up" of the gear clearances and bearings.

Our '69 Dart, originally a 273 car with a 7 1/4-inch rear, had been updated with a later A-Body 8 1/4-inch rearend. With its 5 on 4 1/2-inch wheel bolt pattern, availability, and greater torque handling capacity, it was a good swap. Unfortunately, this rear, like most, was a peg leg. With a built 318 up front, and a four-speed in the tunnel, dropping the hammer produced more tire smoke than acceleration. Considering the remedy, we weighed the options. The rear could be torn down and a new OE-style limited-slip case installed, but the down time and setup hassle made us put it off. Then we heard about the Powertrax.

2/15The Powertrax is a component kit, which replaces the gears in a stock differential case with a locking differential. An inner coupler replaces the side gear and splines onto the axleshaft. The coupler's straight-cut drive teeth on its face side match the teeth on the driver, which is pinned to the case with a heavy shaft. An active spacer between the two works through a syncro system to allow the differential to lock and unlock.

The Powertrax unit is a no-slip retrofit differential designed to fit within the confines of the stock differential case. Make no mistake about it-the Powertrax is a locking differential which results in uncompromised torque splitting when locked, and then automatically unlocks when going around a corner in a normal manner. Simply put, it's either locked or unlocked, and therein lies the caveat. There is a mechanical shift from locked to unlocked and back again. Around a corner, the differential will unlock to disengage the outside wheel under mild acceleration or disengage the inside wheel when coasting or braking. The rear and chassis will react to the change. With the Powertrax unit, extra care is required under certain driving conditions, particularly under hard braking or acceleration in corners and any time the vehicle is driven on wet or slippery roads. Like any locker or limited slip, hammering the throttle in a corner will lock the differential, and the rear will want to come around.

Since it fits in the stock "open" differential case, installing the Powertrax requires only stripping the components out of the differential and installing the Powertrax parts in their place. Because the original case is used, the Powertrax can be installed without fully disassembling the rearend and resetting the gears. It seemed like the ideal upgrade for our street Dart. The installation proved to be as simple and quick as we expected, requiring no special tools, and we were back on the road in a couple of hours. Though we could hear it and feel it while driving, the added straight-line performance made it a worthwhile upgrade.